/*
 * Copyright (c) 2018-2020 "Graph Foundation,"
 * Graph Foundation, Inc. [https://graphfoundation.org]
 *
 * This file is part of ONgDB.
 *
 * ONgDB is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
/*
 * Copyright (c) 2002-2020 "Neo4j,"
 * Neo4j Sweden AB [http://neo4j.com]
 *
 * This file is part of Neo4j.
 *
 * Neo4j is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package org.neo4j.helpers.collection;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Wraps the given iterator but keeps the first item to allow later
 * access to it, like CachingIterator but with less memory overhead.
 *
 * @param <T> the type of elements
 */
public class FirstItemIterable<T> implements Iterable<T>
{
    private final T first;
    private final Iterator<T> iterator;
    private int pos = -1;

    public FirstItemIterable( Iterable<T> data )
    {
        this( data.iterator() );
    }

    public FirstItemIterable( Iterator<T> iterator )
    {
        this.iterator = iterator;
        if ( iterator.hasNext() )
        {
            this.first = iterator.next();
            this.pos = 0;
        }
        else
        {
            this.first = null;
        }
    }

    @Override
    public Iterator<T> iterator()
    {
        return new Iterator<T>()
        {
            @Override
            public boolean hasNext()
            {
                return pos == 0 || iterator.hasNext();
            }

            @Override
            public T next()
            {
                if ( pos < 0 )
                {
                    throw new NoSuchElementException();
                }
                return pos++ == 0 ? first : iterator.next();
            }

            @Override
            public void remove()
            {

            }
        };
    }

    public T getFirst()
    {
        return first;
    }
}
